Three-dimensional virtual histology of the human hippocampus based on phase-contrast computed tomography.

We have studied the three-dimensional (3D) cytoarchitecture of the human hippocampus in neuropathologically healthy and Alzheimer's disease (AD) individuals, based on phase-contrast X-ray computed tomography of postmortem human tissue punch biopsies. In view of recent findings suggesting a nuclear origin of AD, we target in particular the nuclear structure of the dentate gyrus (DG) granule cells. Tissue samples of 20 individuals were scanned and evaluated using a highly automated approach of measurement and analysis, combining multiscale recordings, optimized phase retrieval, segmentation by machine learning, representation of structural properties in a feature space, and classification based on the theory of optimal transport. Accordingly, we find that the prototypical transformation between a structure representing healthy granule cells and the pathological state involves a decrease in the volume of granule cell nuclei, as well as an increase in the electron density and its spatial heterogeneity. The latter can be explained by a higher ratio of heterochromatin to euchromatin. Similarly, many other structural properties can be derived from the data, reflecting both the natural polydispersity of the hippocampal cytoarchitecture between different individuals in the physiological context and the structural effects associated with AD pathology.

Concurrent axon and myelin destruction differentiates X-linked adrenoleukodystrophy from multiple sclerosis.

Cerebral disease manifestation occurs in about two thirds of males with X-linked adrenoleukodystrophy (CALD) and is fatally progressive if left untreated. Early histopathologic studies categorized CALD as an inflammatory demyelinating disease, which led to repeated comparisons to multiple sclerosis (MS). The aim of this study was to revisit the relationship between axonal damage and myelin loss in CALD. We applied novel immunohistochemical tools to investigate axonal damage, myelin loss and myelin repair in autopsy brain tissue of eight CALD and 25 MS patients. We found extensive and severe acute axonal damage in CALD already in prelesional areas defined by microglia loss and relative myelin preservation. In contrast to MS, we did not observe selective phagocytosis of myelin, but a concomitant decay of the entire axon-myelin unit in all CALD lesion stages. Using a novel marker protein for actively remyelinating oligodendrocytes, breast carcinoma-amplified sequence (BCAS) 1, we show that repair pathways are activated in oligodendrocytes in CALD. Regenerating cells, however, were affected by the ongoing disease process. We provide evidence that-in contrast to MS-selective myelin phagocytosis is not characteristic of CALD. On the contrary, our data indicate that acute axonal injury and permanent axonal loss are thus far underestimated features of the disease that must come into focus in our search for biomarkers and novel therapeutic approaches.

Chronic White Matter Inflammation and Serum Neurofilament Levels in Multiple Sclerosis.

OBJECTIVE: To assess whether chronic white matter inflammation in patients with multiple sclerosis (MS) as detected in vivo by paramagnetic rim MRI lesions (PRLs) is associated with higher serum neurofilament light chain (sNfL) levels, a marker of neuroaxonal damage. METHODS: In 118 patients with MS with no gadolinium-enhancing lesions or recent relapses, we analyzed 3D-submillimeter phase MRI and sNfL levels. Histopathologic evaluation was performed in 25 MS lesions from 20 additional autopsy MS cases. RESULTS: In univariable analyses, participants with ≥2 PRLs (n = 43) compared to those with ≤1 PRL (n = 75) had higher age-adjusted sNfL percentiles (median, 91 and 68; p < 0.001) and higher Multiple Sclerosis Severity Scale scores (MSSS median, 4.3 and 2.4; p = 0.003). In multivariable analyses, sNfL percentile levels were higher in PRLs ≥2 cases (ßadd, 16.3; 95% confidence interval [CI], 4.6-28.0; p < 0.01), whereas disease-modifying treatment (DMT), Expanded Disability Status Scale (EDSS) score, and T2 lesion load did not affect sNfL. In a similar model, sNfL percentile levels were highest in cases with ≥4 PRLs (n = 30; ßadd, 30.4; 95% CI, 15.6-45.2; p < 0.01). Subsequent multivariable analysis revealed that PRLs ≥2 cases also had higher MSSS (ßadd, 1.1; 95% CI, 0.3-1.9; p < 0.01), whereas MSSS was not affected by DMT or T2 lesion load. On histopathology, both chronic active and smoldering lesions exhibited more severe acute axonal damage at the lesion edge than in the lesion center (edge vs center: p = 0.004 and p = 0.0002, respectively). CONCLUSION: Chronic white matter inflammation was associated with increased levels of sNfL and disease severity in nonacute MS, suggesting that PRL contribute to clinically relevant, inflammation-driven neurodegeneration.

Three-dimensional virtual histology of the cerebral cortex based on phase-contrast X-ray tomography.

In this work, we optimize the setups and experimental parameters of X-ray phase-contrast computed-tomography for the three-dimensional imaging of the cyto- and myeloarchitecture of cerebral cortex, including both human and murine tissue. We present examples for different optical configurations using state-of-the art synchrotron instruments for holographic tomography, as well as compact laboratory setups for phase-contrast tomography in the direct contrast (edge-enhancement) regime. Apart from unstained and paraffin-embedded tissue, we tested hydrated tissue, as well as heavy metal stained and resin-embedded tissue using two different protocols. Further, we show that the image quality achieved allows to assess the neuropathology of multiple sclerosis in a biopsy sample collected during surgery.

Microglia facilitate repair of demyelinated lesions via post-squalene sterol synthesis.

The repair of inflamed, demyelinated lesions as in multiple sclerosis (MS) necessitates the clearance of cholesterol-rich myelin debris by microglia/macrophages and the switch from a pro-inflammatory to an anti-inflammatory lesion environment. Subsequently, oligodendrocytes increase cholesterol levels as a prerequisite for synthesizing new myelin membranes. We hypothesized that lesion resolution is regulated by the fate of cholesterol from damaged myelin and oligodendroglial sterol synthesis. By integrating gene expression profiling, genetics and comprehensive phenotyping, we found that, paradoxically, sterol synthesis in myelin-phagocytosing microglia/macrophages determines the repair of acutely demyelinated lesions. Rather than producing cholesterol, microglia/macrophages synthesized desmosterol, the immediate cholesterol precursor. Desmosterol activated liver X receptor (LXR) signaling to resolve inflammation, creating a permissive environment for oligodendrocyte differentiation. Moreover, LXR target gene products facilitated the efflux of lipid and cholesterol from lipid-laden microglia/macrophages to support remyelination by oligodendrocytes. Consequently, pharmacological stimulation of sterol synthesis boosted the repair of demyelinated lesions, suggesting novel therapeutic strategies for myelin repair in MS.

Lack of astrocytes hinders parenchymal oligodendrocyte precursor cells from reaching a myelinating state in osmolyte-induced demyelination.

Demyelinated lesions in human pons observed after osmotic shifts in serum have been referred to as central pontine myelinolysis (CPM). Astrocytic damage, which is prominent in neuroinflammatory diseases like neuromyelitis optica (NMO) and multiple sclerosis (MS), is considered the primary event during formation of CPM lesions. Although more data on the effects of astrocyte-derived factors on oligodendrocyte precursor cells (OPCs) and remyelination are emerging, still little is known about remyelination of lesions with primary astrocytic loss. In autopsy tissue from patients with CPM as well as in an experimental model, we were able to characterize OPC activation and differentiation. Injections of the thymidine-analogue BrdU traced the maturation of OPCs activated in early astrocyte-depleted lesions. We observed rapid activation of the parenchymal NG2+ OPC reservoir in experimental astrocyte-depleted demyelinated lesions, leading to extensive OPC proliferation. One week after lesion initiation, most parenchyma-derived OPCs expressed breast carcinoma amplified sequence-1 (BCAS1), indicating the transition into a pre-myelinating state. Cells derived from this early parenchymal response often presented a dysfunctional morphology with condensed cytoplasm and few extending processes, and were only sparsely detected among myelin-producing or mature oligodendrocytes. Correspondingly, early stages of human CPM lesions also showed reduced astrocyte numbers and non-myelinating BCAS1+ oligodendrocytes with dysfunctional morphology. In the rat model, neural stem cells (NSCs) located in the subventricular zone (SVZ) were activated while the lesion was already partially repopulated with OPCs, giving rise to nestin+ progenitors that generated oligodendroglial lineage cells in the lesion, which was successively repopulated with astrocytes and remyelinated. These nestin+ stem cell-derived progenitors were absent in human CPM cases, which may have contributed to the inefficient lesion repair. The present study points to the importance of astrocyte-oligodendrocyte interactions for remyelination, highlighting the necessity to further determine the impact of astrocyte dysfunction on remyelination inefficiency in demyelinating disorders including MS.

Neuropilin-1 facilitates SARS-CoV-2 cell entry and infectivity.

The causative agent of coronavirus disease 2019 (COVID-19) is the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). For many viruses, tissue tropism is determined by the availability of virus receptors and entry cofactors on the surface of host cells. In this study, we found that neuropilin-1 (NRP1), known to bind furin-cleaved substrates, significantly potentiates SARS-CoV-2 infectivity, an effect blocked by a monoclonal blocking antibody against NRP1. A SARS-CoV-2 mutant with an altered furin cleavage site did not depend on NRP1 for infectivity. Pathological analysis of olfactory epithelium obtained from human COVID-19 autopsies revealed that SARS-CoV-2 infected NRP1-positive cells facing the nasal cavity. Our data provide insight into SARS-CoV-2 cell infectivity and define a potential target for antiviral intervention.

Molecular signature of slowly expanding lesions in progressive multiple sclerosis.

Multiple sclerosis is an immune-mediated chronic inflammatory disease of the CNS that leads to demyelinated lesions in the grey and white matter. Inflammatory, active demyelinating white matter lesions predominate in the relapsing-remitting disease stages, whereas in the progressive stage the so-called slowly expanding lesion is characteristic. These lesions show an accumulation of macrophages/microglia at their borders, mediating the ongoing myelin breakdown and axonal degeneration. The exact pathogenetic mechanisms of lesion progression in chronic multiple sclerosis are still not clear. In the present study, we performed a detailed immunological and molecular profiling of slowly expanding lesions (n = 21) from 13 patients aged between 30 to 74 years (five females and eight males), focusing on macrophage/microglia differentiation. By applying the microglia-specific marker TMEM119, we demonstrate that cells accumulating at the lesion edge almost exclusively belonged to the microglia lineage. Macrophages/microglia can be subdivided into the M1 type, which are associated with inflammatory and degenerative processes, and M2 type, with protective properties, whereby also intermediate polarization phenotypes can be observed. By using a panel of markers characterizing M1- or M2-type macrophages/microglia, we observed a preferential accumulation of M1-type differentiated cells at the lesion edge, indicating a crucial role of these cells in lesion progression. Additionally, unbiased RNA microarray analyses of macrodissected lesion edges from slowly expanding and chronic inactive lesions as well as normal-appearing white matter were performed. In slowly expanding lesions, we identified a total of 165 genes that were upregulated and 35 genes that were downregulated. The upregulated genes included macrophage/microglia-associated genes involved in immune defence and inflammatory processes. Among the upregulated genes were ALOX15B, MME and TNFRSF25. We confirmed increased expression of ALOX15B by quantitative PCR, and of all three genes on the protein level by immunohistochemistry. In conclusion, the present study characterized in detail slowly expanding lesions in progressive multiple sclerosis and demonstrated a preferential accumulation of resident microglia with M1 differentiation at the lesion edge. Microarray analysis showed an increased expression of genes related to immune function, metabolic processes as well as transcription/translation. Thus, these genes may serve as future therapeutic targets to impede lesion progression.

Phase-contrast x-ray tomography of neuronal tissue at laboratory sources with submicron resolution.

Purpose: Recently, progress has been achieved in implementing phase-contrast tomography of soft biological tissues at laboratory sources. This opens up opportunities for three-dimensional (3-D) histology based on x-ray computed tomography ( µ - and nanoCT) in the direct vicinity of hospitals and biomedical research institutions. Combining advanced x-ray generation and detection techniques with phase reconstruction algorithms, 3-D histology can be obtained even of unstained tissue of the central nervous system, as shown, for example, for biopsies and autopsies of human cerebellum. Depending on the setup, i.e., source, detector, and geometric parameters, laboratory-based tomography can be implemented at very different sizes and length scales. We investigate the extent to which 3-D histology of neuronal tissue can exploit the cone-beam geometry at high magnification M using a nanofocus transmission x-ray tube (nanotube) with a 300 nm minimal spot size (Excillum), combined with a single-photon counting camera. Tightly approaching the source spot with the biopsy punch, we achieve high M ≈ 10 1 - 10 2 , high flux density, and exploit the superior efficiency of this detector technology. Approach: Different nanotube configurations such as spot size and flux, M , as well as exposure time, Fresnel number, and coherence are varied and selected in view of resolution, field of view, and/or phase-contrast requirements. Results: The data show that the information content for the cytoarchitecture is enhanced by the phase effect. Comparison of results to those obtained at a microfocus rotating-anode x-ray tomography setup with a high-resolution detector, i.e., in low- M geometry, reveals similar to slightly superior data quality for the nanotube setup. In addition to its compactness, reduced power consumption by a factor of 10 3 , and shorter scan duration, the particular advantage of the nanotube setup also lies in its suitability for pixel detector technology, enabling an increased range of opportunities for applications in laboratory phase-contrast x-ray tomography. Conclusions: The phase retrieval scheme utilized mixes amplitude and phase contrast, with results being robust with respect to reconstruction parameters. Structural information content is comparable to slightly superior to previous results achieved with a microfocus rotating-anode setup but can be obtained in shorter scan time. Beyond advantages as compactness, lowered power consumption, and flexibility, the nanotube setup's scalability in view of the progress in pixel detector technology is particularly beneficial. Further progress is thus likely to bring 3-D virtual histology to the performance in scan time and throughput required for clinical practice in neuropathology.

Pro-inflammatory activation following demyelination is required for myelin clearance and oligodendrogenesis.

Remyelination requires innate immune system function, but how exactly microglia and macrophages clear myelin debris after injury and tailor a specific regenerative response is unclear. Here, we asked whether pro-inflammatory microglial/macrophage activation is required for this process. We established a novel toxin-based spinal cord model of de- and remyelination in zebrafish and showed that pro-inflammatory NF-κB-dependent activation in phagocytes occurs rapidly after myelin injury. We found that the pro-inflammatory response depends on myeloid differentiation primary response 88 (MyD88). MyD88-deficient mice and zebrafish were not only impaired in the degradation of myelin debris, but also in initiating the generation of new oligodendrocytes for myelin repair. We identified reduced generation of TNF-α in lesions of MyD88-deficient animals, a pro-inflammatory molecule that was able to induce the generation of new premyelinating oligodendrocytes. Our study shows that pro-inflammatory phagocytic signaling is required for myelin debris degradation, for inflammation resolution, and for initiating the generation of new oligodendrocytes.

Nanoscale x-ray holotomography of human brain tissue with phase retrieval based on multienergy recordings.

X-ray cone-beam holotomography of unstained tissue from the human central nervous system reveals details down to subcellular length scales. This visualization of variations in the electron density of the sample is based on phase-contrast techniques using intensities formed by self-interference of the beam between object and detector. Phase retrieval inverts diffraction and overcomes the phase problem by constraints such as several measurements at different Fresnel numbers for a single projection. Therefore, the object-to-detector distance (defocus) can be varied. However, for cone-beam geometry, changing defocus changes magnification, which can be problematic in view of image processing and resolution. Alternatively, the photon energy can be altered (multi-E). Far from absorption edges, multi-E data yield the wavelength-independent electron density. We present the multi-E holotomography at the Göttingen Instrument for Nano-Imaging with X-Rays (GINIX) setup of the P10 beamline at Deutsches Elektronen-Synchrotron. The instrument is based on a combined optics of elliptical mirrors and an x-ray waveguide positioned in the focal plane for further coherence, spatial filtering, and high numerical aperture. Previous results showed the suitability of this instrument for nanoscale tomography of unstained brain tissue. We demonstrate that upon energy variation, the focal spot is stable enough for imaging. To this end, a double-crystal monochromator and automated alignment routines are required. Three tomograms of human brain tissue were recorded and jointly analyzed using phase retrieval based on the contrast transfer function formalism generalized to multiple photon energies. Variations of the electron density of the sample are successfully reconstructed.

Correlative x-ray phase-contrast tomography and histology of human brain tissue affected by Alzheimer's disease.

Alzheimer's disease (AD) is a neurodegenerative disorder which is characterized by increasing dementia. It is accompanied by the development of extracellular ß-amyloid plaques and neurofibrillary tangles in the gray matter of the brain. Histology is the gold standard for the visualization of this pathology, but also has intrinsic shortcomings. Fully three-dimensional analysis and quantitative metrics of alterations in the tissue structure require a complementary approach. In this work we use x-ray phase-contrast tomography to obtain three-dimensional reconstructions of human hippocampal tissue affected by AD. Due to intrinsic electron density differences, tissue components and structures such as the granule cells of the dentate gyrus, blood vessels, or mineralized plaques can be identified and segmented in large volumes. Based on correlative histology, protein (tau, ß-amyloid) and elemental content (iron, calcium) can be attributed to certain morphological features occurring in the entire volume. In the vicinity of senile plaques, an accumulation of microglia in combination with a loss of neuronal cells can be observed.

Increased HLA-DR expression and cortical demyelination in MS links with HLA-DR15.

OBJECTIVE: To investigate molecular changes in multiple sclerosis (MS) normal-appearing cortical gray matter (NAGM). METHODS: We performed a whole-genome gene expression microarray analysis of human brain autopsy tissues from 64 MS NAGM samples and 42 control gray matter samples. We further examined our cases by HLA genotyping and performed immunohistochemical and immunofluorescent analysis of all human brain tissues. RESULTS: HLA-DRB1 is the transcript with highest expression in MS NAGM with a bimodal distribution among the examined cases. Genotyping revealed that every case with the MS-associated HLA-DR15 haplotype also shows high HLA-DRB1 expression and also of the tightly linked HLA-DRB5 allele. Quantitative immunohistochemical analysis confirmed the higher expression of HLA-DRB1 in HLA-DRB1*15:01 cases at the protein level. Analysis of gray matter lesion size revealed a significant increase of cortical lesion size in cases with high HLA-DRB1 expression. CONCLUSIONS: Our data indicate that increased HLA-DRB1 and -DRB5 expression in the brain of patients with MS may be an important factor in how the HLA-DR15 haplotype contributes to MS pathomechanisms in the target organ.

Cross-Species Single-Cell Analysis Reveals Divergence of the Primate Microglia Program.

Microglia, the brain-resident immune cells, are critically involved in many physiological and pathological brain processes, including neurodegeneration. Here we characterize microglia morphology and transcriptional programs across ten species spanning more than 450 million years of evolution. We find that microglia express a conserved core gene program of orthologous genes from rodents to humans, including ligands and receptors associated with interactions between glia and neurons. In most species, microglia show a single dominant transcriptional state, whereas human microglia display significant heterogeneity. In addition, we observed notable differences in several gene modules of rodents compared with primate microglia, including complement, phagocytic, and susceptibility genes to neurodegeneration, such as Alzheimer's and Parkinson's disease. Our study provides an essential resource of conserved and divergent microglia pathways across evolution, with important implications for future development of microglia-based therapies in humans.

GM-CSF and CXCR4 define a T helper cell signature in multiple sclerosis.

Cytokine dysregulation is a central driver of chronic inflammatory diseases such as multiple sclerosis (MS). Here, we sought to determine the characteristic cellular and cytokine polarization profile in patients with relapsing-remitting multiple sclerosis (RRMS) by high-dimensional single-cell mass cytometry (CyTOF). Using a combination of neural network-based representation learning algorithms, we identified an expanded T helper cell subset in patients with MS, characterized by the expression of granulocyte-macrophage colony-stimulating factor and the C-X-C chemokine receptor type 4. This cellular signature, which includes expression of very late antigen 4 in peripheral blood, was also enriched in the central nervous system of patients with relapsing-remitting multiple sclerosis. In independent validation cohorts, we confirmed that this cell population is increased in patients with MS compared with other inflammatory and non-inflammatory conditions. Lastly, we also found the population to be reduced under effective disease-modifying therapy, suggesting that the identified T cell profile represents a specific therapeutic target in MS.

Brain-resident memory T cells generated early in life predispose to autoimmune disease in mice.

Epidemiological studies associate viral infections during childhood with the risk of developing autoimmune disease during adulthood. However, the mechanistic link between these events remains elusive. We report that transient viral infection of the brain in early life, but not at a later age, precipitates brain autoimmune disease elicited by adoptive transfer of myelin-specific CD4+ T cells at sites of previous infection in adult mice. Early-life infection of mouse brains imprinted a chronic inflammatory signature that consisted of brain-resident memory T cells expressing the chemokine (C-C motif) ligand 5 (CCL5). Blockade of CCL5 signaling via C-C chemokine receptor type 5 prevented the formation of brain lesions in a mouse model of autoimmune disease. In mouse and human brain, CCL5+ TRM were located predominantly to sites of microglial activation. This study uncovers how transient brain viral infections in a critical window in life might leave persisting chemotactic cues and create a long-lived permissive environment for autoimmunity.

Microglia damage precedes major myelin breakdown in X-linked adrenoleukodystrophy and metachromatic leukodystrophy.

X-linked adrenoleukodystrophy (X-ALD) and metachromatic leukodystrophy (MLD) are two relatively common examples of hereditary demyelinating diseases caused by a dysfunction of peroxisomal or lysosomal lipid degradation. In both conditions, accumulation of nondegraded lipids leads to the destruction of cerebral white matter. Because of their high lipid content, oligodendrocytes are considered key to the pathophysiology of these leukodystrophies. However, the response to allogeneic stem cell transplantation points to the relevance of cells related to the hematopoietic lineage. In the present study, we aimed to better characterize the pathogenetic role of microglia in the above-mentioned diseases. Applying recently established microglia markers to human autopsy cases of X-ALD and MLD we were able to delineate distinct lesion stages in evolving demyelinating lesions. The immune-phenotype of microglia was altered already early in lesion evolution, and microglia loss preceded full-blown myelin degeneration both in X-ALD and MLD. DNA fragmentation indicating phagocyte death was observed in areas showing microglia loss. The morphology and dynamics of phagocyte decay differed between the diseases and between lesion stages, hinting at distinct pathways of programmed cell death. In summary, the present study shows an early and severe damage to microglia in the pathogenesis of X-ALD and MLD. This hints at a central pathophysiologic role of these cells in the diseases and provides evidence for an ongoing transfer of toxic substrates primarily enriched in myelinating cells to microglia.

Three-dimensional virtual histology of human cerebellum by X-ray phase-contrast tomography.

To quantitatively evaluate brain tissue and its corresponding function, knowledge of the 3D cellular distribution is essential. The gold standard to obtain this information is histology, a destructive and labor-intensive technique where the specimen is sliced and examined under a light microscope, providing 3D information at nonisotropic resolution. To overcome the limitations of conventional histology, we use phase-contrast X-ray tomography with optimized optics, reconstruction, and image analysis, both at a dedicated synchrotron radiation endstation, which we have equipped with X-ray waveguide optics for coherence and wavefront filtering, and at a compact laboratory source. As a proof-of-concept demonstration we probe the 3D cytoarchitecture in millimeter-sized punches of unstained human cerebellum embedded in paraffin and show that isotropic subcellular resolution can be reached at both setups throughout the specimen. To enable a quantitative analysis of the reconstructed data, we demonstrate automatic cell segmentation and localization of over 1 million neurons within the cerebellar cortex. This allows for the analysis of the spatial organization and correlation of cells in all dimensions by borrowing concepts from condensed-matter physics, indicating a strong short-range order and local clustering of the cells in the granular layer. By quantification of 3D neuronal "packing," we can hence shed light on how the human cerebellum accommodates 80% of the total neurons in the brain in only 10% of its volume. In addition, we show that the distribution of neighboring neurons in the granular layer is anisotropic with respect to the Purkinje cell dendrites.

BCAS1 expression defines a population of early myelinating oligodendrocytes in multiple sclerosis lesions.

Investigations into brain function and disease depend on the precise classification of neural cell types. Cells of the oligodendrocyte lineage differ greatly in their morphology, but accurate identification has thus far only been possible for oligodendrocyte progenitor cells and mature oligodendrocytes in humans. We find that breast carcinoma amplified sequence 1 (BCAS1) expression identifies an oligodendroglial subpopulation in the mouse and human brain. These cells are newly formed, myelinating oligodendrocytes that segregate from oligodendrocyte progenitor cells and mature oligodendrocytes and mark regions of active myelin formation in development and in the adult. We find that BCAS1+ oligodendrocytes are restricted to the fetal and early postnatal human white matter but remain in the cortical gray matter until old age. BCAS1+ oligodendrocytes are reformed after experimental demyelination and found in a proportion of chronic white matter lesions of patients with multiple sclerosis (MS) even in a subset of patients with advanced disease. Our work identifies a means to map ongoing myelin formation in health and disease and presents a potential cellular target for remyelination therapies in MS.